1. Field of the Invention
The present invention relates to a substance vaporizing apparatus for generating a vapor of a substance to be used as an ionizing medium or an exciting medium and, more specifically, to a structure capable of functioning to vaporize a substance.
2. Description of the Prior Art
A metal vapor laser and a vapor ion laser incorporate such a substance vaporizing apparatus. FIG. 1 shows a conventional metal vapor laser is published in, for example, 6th Nenji Taikai Koen Yoko-shu, 21aIIB3, pp. 60-63 (1986). As shown in FIG. 1, the metal vapor laser comprises discharge electrodes 1a and 1b, a cylindrical discharge tube 2 defining a discharge space 3 in which a metal vapor 5 produced by vaporizing metal pieces 4, such as copper pieces, is excited, a heat shielf 6 formed of a heat-insulating material, such as wool, resonant mirrors 7a and 7b for laser oscillation, flanges 8a and 8b defining a sealed space including a vacuum chamber 9, an insulating tube 10, a sealing tube 11, a gas inlet port 12a, and a gas outlet port 12b.
In operation, a pulse voltage is applied across the electrodes 1a and 1b to form a discharge within the discharge space 3, and then a buffer gas filling the discharge space 3 is heated by the energy of accelerated ions and electrons to vaporize the metal pieces 4. The ions and electrons accelerated by a pulse discharge and having high energy and high-temperature atoms of the heated buffer gas collide against the atoms of the vaporized metal to transfer their energy to the atoms of the vaporized metal, so that the atoms of the vaporized metal is excited to a higher energy level. The heat shield 6 serves for maintaining the gas at a desired temperature to maintain a predetermined vapor density in the discharge space 3. The function of the vacuum chamber 9 is similar to that of the heat shield; the vacuum chamber 9 suppresses particularly convection heat loss. Light is emitted when the energy level of the atoms of the vaporized metal drops to a lower energy level or the ground energy level.
The light thus emitted is amplified optically by the resonant mirrors 7a and 7b to emit a laser beam outside for industrial uses, such as laser machining.
FIG. 2 shows the buffer gas temperature distribution and the vapor density distribution with respect to the diametrical and axial directions of the discharge space 3 in the metal vapor laser thus constructed. In FIG. 2, X-axis and Y-axis coincide respectively with the diametrical direction and the axial direction of the discharge space 3; temperature is measured upward on T-axis; indicated at 3a is the center axis of the discharge space 3 and at 3b are diametrically opposite ends of the discharge space. A curve l represented by a dotted line indicates temperature distribution with respect to the axial direction, curves n.sub.1, n.sub.2 and n.sub.3 represented by alternate long and short dash line indicate vapor distribution with respect to the diametrical direction, and curves m.sub.1, m.sub.2 and m.sub.3 represented by continuous lines indicated temperature distribution with respect to the diametrical direction. As seen in FIG. 2, the temperature of the buffer gas decreases from the central portion of the discharge space 3 around the center axis 3a toward the periphery of the discharge space 3 represented by the diametrically opposite ends 3b. The temperature of the buffer gas in the vicinity of the axial ends of the discharge space 3 is lower than that in the central portion of the same. Since the vapor density distribution within the discharge tube 2 can be approximated by saturation vapor density n.sub.0, which is a function of buffer gas temperature, the relation between the vapor density distribution curves n.sub.1, n.sub.2 and n.sub.3 is expressed by: n.sub.1 &gt;n.sub.2 =n.sub.3.
On the other hand, when the vapor density increases relative to the buffer gas density (buffer gas pressure), the mean free path of electrons, and the ions and neutral atoms of the buffer gas decreases, and hence the kinetic energy acquired by the electrons and ions from the electric field created by a pulse discharge before colliding against the atoms of the vapor decreases and when the temperature of the atoms of the vapor increase too much, the atoms of the vapor of lower energy level increase, therefore, the population invertion between the number of atoms of the vapor of a higher energy level and that of atoms of the vapor of a lower one is not created, consequently, the electrons and the ions are unable to excite the atoms of the vapor to an energy level high enough to a give a sufficient laser gain. This conventional metal vapor laser has a disadvantage that the vapor density is high in the central portion of the discharge space 3 and hence the laser power density is low in the central portion of the discharge space 3.
FIG. 3 shows a copper vapor laser described in "Manufacture of Copper Vapor Laser" published in "Reza Kenkyu", pp. 60-66, March 1981.
Referring to FIG. 3, a vapor laser unit 100 comprises opposite electrodes 1 for forming a discharge in a gas, a discharge tube 2 containing copper particles 4, a sealing tube 11, a heat shield 6 for preventing the loss of heat generated by a discharge formed between the electrodes 1, windows 13 disposed respectively near the electrodes 1 to emit a laser beam, and an insulating break 21 provided on the sealing tube 11 for high-voltage insulation.
A first pulse circuit 200 comprises a charging capacitor 14 connected to the sealing tube 11 of the vapor laser unit 100 by a connecting line a, a charging reactor 15 connected in series to the charging capacitor 14, a high-voltage power source 17, a diode 16 having a positive electrode connected to the high-voltage power source 17 and a negative electrode connected to the charging reactor 15, a thyratron 18 connected to the charging reactor 15 and the high-voltage power source 17, and a peaking capacitor 20 connected to the charging capacitor 14 and the thyratron 18. The high-voltage power source 17 is connected to the sealing tube 11 by a connecting line b.
The charging capacitor 14 is charged at a high voltage through the diode 16, the charging reactor 15 and the peaking capacitor 20 by the high-voltage power source 17.
When the thyratron 18 is turned on by a pulse control circuit 19, the charging capacitor 14 is charged to apply a high voltage through the sealing tube 11 across the electrodes 1 to form a discharge in the discharge tube 2. Since the heat shield 6 prevents the dissipation of the thermal energy generated by the discharge within the discharge tube, the temperature within the discharge tube 2 is raised to a high temperature on the order of 1500.degree. C., whereby the copper particles 4 are vaporized and the discharge tube 2 is filled with copper vapor.
Electrons of a plasma accelerated by the discharge formed between the opposite electodes 1 collide against copper atoms filling the discharge tube 2 to excite the copper atoms to a higher energy level corresponding to a first resonance level. Since less atoms are excited to a lower energy level corresponding to a metastable energy level, a population inversion is created. The energy level of the copper atoms drops from the higher energy level to a lower energy level with laser oscillation, and then drops gradually from the lower energy level to the ground energy level. This cycle is repeated at a frequency of several kilohertzs. The laser beam is emitted through the windows 13.
The drop of the energy level from the lower energy level to the ground energy level is caused by the collision of the excited atoms against the wall of the discharge tube 2 when the discharge tube has a relatively small diameter, and is caused by the superelastic collision of the atoms of the lower energy level and the slow electrons when the discharge tube 2 has a relatively large diameter, and the lifetime of the atoms on the lower energy level is as long as several hundreds microseconds.
When the high-voltage pulse is applied across the electrodes 1 at a long interval corresponding to the long time of several hundreds microseconds required for the lower energy level to drop to the ground energy level, the frequency of high-voltage pulse application is reduced. On the other hand, atoms of the lower energy level increases when the high-voltage pulse is applied at a high frequency, so that an incomplete population inversion is created and hence the copper vapor laser is unable to operate at a high efficiency.